CLASSIFICATION 483 



solitary embryos which develop into solitary Salps. This is 

 the puzzling phenomenon of the alternation of generations — 

 the alternate occurrence in one life-history of two or more 

 different forms differently produced, and it is interesting to 

 study the phenomenon where it was first discovered (by the 

 poet Chamisso), in the Salps. 



At the present time it seems legitimate to say that just as 

 the Clavelinidae may have linked the Ascidiae Simplices to the 

 Ascidiae Compositae, so the Thaliacea may have arisen from a 

 stock of ancestral Compound Ascidians which gave rise to the 

 Pyrosomes. 



Compared with many others the class of Tunicates is 

 a small one, but over a thousand species are now known. In 

 Herdman's Revised Classification of the Tunicata (1891) there 

 was a record of 538 species, and the list has been almost 

 doubled since, mainly by the exploration of new areas. The 

 great majority of the additions made to the roll of Tunicates 

 since 1891 are sedentary forms. 



The number of species often raises very interesting problems. 

 There seem to be variable types which have given origin to 

 many species, and conservative types which have few specific 

 representatives. In the case of sedentary Tunicates a large 

 number of species belong to a few genera or groups of genera. 

 In this connection Professor W. E. Ritter 1 says : " The most 

 conspicuous groups from this standpoint are I, Ascidia; 2, 

 Molgula ; 3, Cynthia with its close ally Rhabdocynthia ; 4, 

 Styela, with the scarcely distinguishable Polycarpa ; 5, Botryllus 

 and its close congener Botrylloides ; 6, Amaroucium with its 

 near relative Aplidium ; and 7, Leptoclinum. These seven 

 groups contain more than 600 of the approximately 1000 species 

 of simple and compound Ascidians now described. There are 

 recognised at least 80 genera in these two Tunicate sections. 

 In other words, as our scheme of classification now stands less 

 than 14 per cent of the genera contain fully 60 per cent of all 

 the species. It will be observed that these few prolific groups 

 present all the leading types of sedentary Ascidian organisation." 

 Ritter goes on to say, " I do not believe there is anything in our 

 present knowledge of Ascidian structure, function, or distribution, 



lu The Significant Results of a Decade's Study of the Tunicata," Amer. 

 Naturalist, xli. 1907, p. 455. 



